1. Field of the Invention
The invention relates to a charged particle accelerator which introduces charged particles into a radio frequency electro-magnetic field for accelerating the charged particles and obtaining a high kinetic energy, in particular relates to a heavy ion accelerator.
2. Description of the Prior Art
FIG. 9 and FIG. 10 show a conceptional construction of the charged particle accelerators with the conventional Radio Frequency Quadrupole (abbreviated as RFQ) type. FIG. 9 shows the H--H cross-section in FIG. 10, and FIG. 10 shows the G--G cross-section in FIG. 9. In these figures, the numeral 1 denotes an accelerating cavity, the numerals 2-5 denote flat electrodes which are arranged along a longitudinal direction of the accelerating cavity 1 and they are also arranged toward four radial directions from the rotationally symmetrical center of the accelerating cavity 1. The numeral 2 denotes the first electrode, and similarly the numeral 3, the second electrode, the numeral 4, the third electrode, and the numeral 5, the fourth electrode. The numeral 6 denotes the beam entrance which is prepared for introducing the charged particles and arranged at one end of the accelerating cavity 1.
The numeral 7 denotes the beam exit which is prepared for emitting the particles arranged at another end of the accelerating cavity 1. The numeral 8 denotes the beam axis which is linearly set up between the beam entrance 6 and the beam exit 7. The numeral 9 denotes a vacuum space which extends in all inner space of the accelerating cavity 1 except the space occupied by the four electrodes 2-5. The numeral 10 denotes a radio frequency coupler which is mounted on the outside of the accelerating cavity 1, and through which a desired radio frequency power is introduced into the accelerating cavity 1.
The conventional charged particle accelerator is constituted as described above. When the radio frequency power of the predetermined resonance frequency f is introduced into the accelerating cavity 1 through the radio frequency coupler 10, both of an electric field for accelerating the charged particles and a quadrupole electric field for preventing the diverging of the charged particles are formed on and near the beam axis 8, as well known by persons skilled in the art. The charged particle introduced from the beam entrance 6 gains a kinetic energy E at the beam exit 7 and are emitted from the beam exit 7.
In the case where the accelerating cavity 1 is a radio frequency quadrupole accelerating cavity, the following relationship is obtained. EQU E.apprxeq.K.multidot.f.sup.2
where, each meaning of symbols is shown in the following.
E: an output kinetic energy of the charged particle at the beam exit 7, PA1 f: a resonance frequency, PA1 K: a constant which depends on the kinds of the charged particle and the construction of the accelerating cavity. PA1 (1) The charged particles are prevented from diverting by equipping one or more magnetic field generation coils (which are called focussing coils in the following) inside or outside of the cavities, PA1 (2) The charged particles are deflected on an off-axis and returned to the beam axis by equipping one or more deflection coils in the cavities, PA1 (3) The kinetic energy of charged particles emitted from the accelerator can be variable by equipping one or more accelerating cavities with both of the focussing and deflection coils. PA1 (1) radio frequency couplers for introducing radio frequency powers into the accelerating cavities, PA1 (2) one or more partition walls which are mounted between two accelerating tubes, and separate one accelerating tube from another neighboring accelerating tube. Each wall has a hole through which the charged particles pass.
The kinetic energy is defined uniquely if the resonance frequency is decided.
Both intensities of the accelerating electric field and the quadrupole electric field depend on the waveforms of the front ends or the electric potentials of the four electrodes 2-5. Only the quadrupole electric field is described below in detail since the electric field is not directly concerned any more with the invention.
That is, the quadrupole electric field acts so as to converge or diverge the charged particles and it is able to transport the charged particles usefully by combining both effects of the convergence and the divergence under a little loss of particles. This quadrupole electric field is obtained, when both of the first electrode 2 and the third electrode 4 are maintained at a plus value in their electric potentials, and moreover the second electrode 3 and the fourth electrode 5 are maintained at a minus value in their electric potentials or vice versa, where the absolute value of the potential of the two electrodes, 3 and 5 is ideally equal to the absolute value of the potential of the two electrodes 2 and 4.
The quadrupole electric field strength seems to be able to increase infinitely with increasing electric potentials of these electrodes. However the strength is limited by electric breakdowns as well known by persons skilled in the art, and cannot be kept more than the strength corresponding to the electric breakdown potential.
As described above, the accelerating electric field and the quadrupole electric field in the charged particle accelerator depend on the wave shape of the front end or the electric potential of the first electrode 2, the second electrode 3, the third electrode 4 and the fourth electrode 5.
The quadrupole electric field having a good rotational symmetry with the beam axis 8 can be generated when the four electrodes 2-4, which are placed every 90.degree. in rotated angle, are arranged at these precise positions.
A very high technique is necessary for arranging the first electrode 2, the second electrode 3, the third electrode 4 and the fourth electrode 5 in these predetermined precise positions. That is, if the first electrode 2, the second electrode 3, the third electrode 4 and the fourth electrode 5 are not arranged at these respective predetermined positions, such an electric field as deflects the charged particles on the beam axis 8, is often generated on and near the beam axis 8. Then a part of the charged particles can not reach the beam exit 7 and they may be lost in the accelerating cavity 1.
When the four electrodes 2-5 are not put within permissible errors together into the cylinder of the accelerating cavity 1, it often is necessary to re-machine and re-assemble in order to obtain the precise arrangement of the electrodes 2-5.
Furthermore, there is another problem that the kinetic energy of emitted charged particles can not be varied in compliance with any demand in the conventional charged particle accelerator as described in the above equation, since the emitted energy of the charged particles becomes nearly constant if the resonance frequency is introduced into the accelerating cavity 1.